In holographic data storage systems (HDSS), a reference beam and an object beam are coincident upon a media suitable for holographic recording. For maximum fringe visibility in the interference fringes produced, these two beams preferably have polarization vectors that are substantially parallel to each other. In most HDSS, this condition requires that the polarization vectors are perpendicular to the plane of incidence defined by the holographic media and are described to be TE-polarized. In order to record multiple co-locational holographic pages in a single location of a holographic media, one multiplexes using a variety of techniques including angle, peristrophic, shift, and/or speckle. Typically in a HDSS, high numerical aperture optics are used for the object beam in order to maximize the achievable storage capacity, and, consequently, high incident angles are required for the reference beams. For page-based optical systems (e.g., holographic optical systems that use spatial light modulators and detectors containing one-dimension (1-D) or two-dimensional (2-D) arrays of pixels), one has a relatively large optical area on the holographic media (few hundred microns) compared to a bit-based system (sub-micron), such as is the case of DVD and CD devices. Therefore, the energy densities where the recording takes place in a DVD and CD media is significantly higher for the same power laser as compared to the energy densities in the media of a page-based HDSS. In the case of DVD and CD recordable material, there is a thermal threshold below which no recording takes place and above which recording can take place. Therefore, scattered light (which is of significantly lower intensity compared to the primary focused spot of the DVD or CD optical system) within a DVD or CD device cannot record erroneous bits in the phase-change material of a DVD or CD. For holographic media, however, materials such as those comprising photopolymerizable reactants do not behave in such a non-linear fashion due to the photointitiation process, and thus can be highly sensitive to scattered or stray light during the holographic recording process. Such reflected scattered or stray light can undesirably expose the holographic media outside the intended exposure area, thereby using up some of the recordable dynamic range of the holographic media. This problem is illustrated in FIG. 1, which represents a cross section of a holographic media 10 along the recording plane as may be the case of an angle-multiplexing HDSS. The holographic media 10 is composed of two substrates 11 that sandwich the photosensitive material 12, as is the case of holographic photopolymerizable materials that are commercially available. An object beam 13 and a reference beam 14 interfere within the photosensitive material and record a hologram in an area 16 represented by the hash marks. A number of rays N of the reference beam, however, also reflect at the substrate-to-air interface, and, consequently, the reflections, depicted in FIG. 1 by the light 15 contained in this ray (and reflecting multiple times as represented by the dashed lines) will expose additional photosensitive material outside of the intended exposure area 16. Such reflections are referred to as Fresnel reflections.
The conventional manner to suppress Fresnel reflections is through the use of thin-film coatings. These antireflection (AR) coatings are typically multi-layer and can be designed for a general material interface, angles of incidence, polarizations, and spectral bandwidth. At least one and preferably two external surfaces 17 of the holographic media containing the holographic material are AR-coated. For example, such AR coated holographic media is sold by Aprilis, Inc. of Maynard, Mass., U.S.A., and U.S. Published Patent Application No. US 2003/0151814, published Aug. 14, 2003, describes the use of AR coatings on holographic media. Although useful for reducing stray or scattered light, they require a thin-film layering process which can have problems with adhesion and thermal expansion mismatches with the media surfaces they are applied to. Further, AR coatings are difficult to apply when such substrates of the media are of plastic, such as polycarbonate. Consequently, an alternative to the use of thin-film coatings is desirable.
Subwavelength structured (SWS) surfaces can be designed for anti-reflection, and is referred to hereinafter as anti-reflection structured (ARS) surfaces. Typically, ARS surfaces contain surface-relief gratings that impedance matches two media where one is a solid, and the other a gas, liquid, or solid. By structuring a surface with a subwavelength-period pattern (either through etching, embossing, or other techniques), an index of refraction distribution can be synthesized so that surface reflections are minimized. When designed properly, these structures can operate over large spectral bandwidths and fields of view. Because foreign materials are not being added to the substrate surface, problems commonly encountered in thin-film technology, such as adhesion and thermal expansion mismatches, are non-existent in the design of these structured surfaces.
Although relatively a new technology, ARS surfaces can be found on the cornea of certain night-flying moths. See, C. G. Bernhard, “Structural and functional adaptation in a visual system,” Endeavor 26, 79–84 (1967). The subwavelength structures of the moth's cornea reduce surface reflections which would otherwise betray the moth's position to its predators. Investigations in ARS surfaces for applications in the visible or near-IR portion of the spectrum have replicated moth-eye surfaces. See, P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the ‘moth eye’ principle,” Nature (London) 244, 281–282 (1973), M. C. Hutley, “Coherent photofabrication,” Opt. Eng. 15, 190–196 (1976); and S. J. Wilson and M. C. Hutley, “The optical properties of ‘moth eye’ antireflection surfaces,” Opt. Acta 29, 993–1009 (1982). These moth-eye structures are an array of 2-D structures that are approximately sinusoidal in shape. Non moth-eye ARS surfaces have also been investigated. See, M. G. Moharam and T. K. Gaylord, “Diffraction analysis of dielectric surface-relief gratings,” J. Opt. Soc. Am. 72, 1385–1392 (1982); Y. Ono, Y. Kimura, Y. Ohta, and N. Nishida, “Antireflection effect in ultrahigh spatial-frequency holographic relief gratings,” Appl. Opt. 26, 1142–1146 (1987); U.S. Pat. No. 5,007,708, issued Apr. 16, 1991; and D. H. Raguin and G. M. Morris, “Antireflection structured surfaces for the infrared spectral region,” Appl. Opt. 32, 1154–1167 (1993). In U.S. Pat. No. 5,007,708, the ARS surfaces described are limited to having binary or staircase profiles. Although ARS surfaces have been proposed by the above literature, they have not been incorporated into holographic data storage media with surface profiles as set forth in the present invention.
FIG. 2 illustrates the reflectivity of a moth eye structure produced by Optical Switch Corporation of Bedford, Mass. for use in a window or screen over an electronic display. The structure had a 260 nm grating period and was replicated (cast-and-cure) into an index-matching polymer on both sides of a BK7 substrate. The figure illustrates the measured double-sided reflectivity of the substrate as a function of angle of incidence (AOI) for both TE and TM polarizations of a 532 nm double-YAG laser. The reflectivity is extremely low and for this particular moth eye design can be reduced to below 0.5% for TE polarization out to about 54°.
Although all holographic media are not composed of multiple distinct materials, current photopolymers marketed for holographic data storage require the sandwiching, as illustrated in FIG. 1, of the photopolymer between two substrates that may be glass or plastic, such as sold by InPhase Technologies of Longmont, Colo., U.S.A., and Aprilis, Inc. of Maynard, Mass., U.S.A. Since several different materials are contained within the holographic media, one requires that the materials bond sufficiently well together, in addition to having means by which Fresnel reflections are suppressed at the various material interfaces (both external and internal) that comprise the holographic media.